Kinetics of NO Reduction by Coal, Biomass, and Graphitic Chars: Effects of Burnout Level and Conditions

Abstract

During the combustion of coal and other carbonaceous materials, the heterogeneous reaction of NO with the evolving char created in the combustion process is important for understanding the formation and reduction of NO. This investigation quantifies the effects of char burnout level and conditions on the kinetics of NO reduction by chars made from coals ranging in rank from lignite to low-volatile bituminous (Beulah Zap, Dietz, Utah Blind Canyon, Pittsburgh #8, and Pocahontas #3) as well as graphite and coconut char. Kinetic data were measured in a packed-bed reactor at temperatures between 723 and 1173 K. The rate constant for the NO−char reaction was found to depend upon the extent of burnout/conversion and conditions under which the char was burned out. The NO−char rate constant consistently decreases with increasing burnout when the char burnout levels are accomplished in a drop tube reactor at 1800 K and 3−5% O 2 . However, the NO−char rate constant increases as char burnout increases (up to 90%) when the char burnout levels result from reacting the char with 3050 ppm of NO in a packed bed at 723−1173 K. For the latter case, the relationship of the NO−char rate constant (on the basis of moisture ash-free char mass) and char burnout is approximately linear with roughly the same upward slope between 20 and 80% burnout for all coal chars studied. The activation energy of the NO−char reaction is apparently independent of both char burnout level and burnout conditions. The CO 2 /CO ratio in the exhaust stream appears to correlate with the NO−char rate constant for the chars burned out at low-temperature conditions.

Models To Predict Kinetics of NOx Reduction by Chars as a Function of Coal Rank

“…In the last 30 years, remarkable progress has been achieved in reducing the concentration of NO x in the atmosphere: nevertheless, some details of the mechanism and kinetics of NO x reduction by coal, biomass, and char remain unresolved. Fossil fuel combustion represents the main source of atmospheric NO x , , the formation of which proceeds through four chemical mechanisms: the Zeldovich (thermal) mechanism, the Fenimore (prompt) mechanism, the N 2 O intermediate mechanism, and the NNH mechanism. − The four mechanisms become dominant under different conditions, for example, the thermal mechanism dominates at high temperatures (>1800 K) with fuels that contain no fuel-bound nitrogen and the intermediate N 2 O mechanism becomes important in low temperatures.…”

“…NO x occurs inherently during coal combustion by three mechanisms: reaction with other NO x molecules, reduction by hydrocarbons, and heterogeneous reduction by char. ,, The mechanism and kinetics of the first two, homogeneous reduction pathways have been studied more thoroughly compared to heterogeneous reduction pathway. Miller and Bowman investigated the Zeldovich (thermal) mechanism and the Fenimore (prompt) mechanism and rate parameters for the gas-phase reactions.…”

Effects of CO 2 and CO on the reduction of NO over calcined limestone or char in oxy‐fuel fluidised bed combustion

“…The NO–char reduction process is assumed as first‐order reaction with respect to NO [16, 17], the NO consumption rate can be derived as where k is the rate constant based on char concentration, mol/(g atm s); C char is the concentration of char particles, g/m 3 ; P NO is the partial pressure of NO, atm.…”

“…The NO-char reduction process is assumed as first-order reaction with respect to NO [16,17], the NO consumption rate can be derived as…”

“…where * represents the active sites on CaO surface. Reactions (14) and (15) are the adsorption of CO and NO on CaO surface, reactions (16) and (17) are the surface reaction, reactions (18) and (19) are the desorption of N 2 and CO 2 . The property of N 2 is very stable, and it is difficult to decompose once generated, so it can be considered that reactions (17) and (18)…”